The program CO2SYS performs calculations relating parameters of the carbon dioxide (CO{sub 2}) system in seawater and freshwater. The program uses two of the four measurable parameters of the CO{sub 2} system [total alkalinity (TA), total inorganic CO{sub 2} (TCO{sub 2}), pH, and either fugacity (fCO{sub 2}) or partial pressure of CO{sub 2} (pCO{sub 2})] to calculate the other two parameters at a set of input conditions (temperature and pressure) and a set of output conditions chosen by the user. It replaces and extends the programs CO2SYSTM.EXE, FCO2TCO2.EXE, PHTCO2.EXE, and CO2BTCH.EXE, which were released in May 1995. It may be run in single-input mode or batch-input mode and has a variety of options for the various constants and parameters used. An on-screen information section is available that includes documentation on various topics relevant to the program. This program may be run on any 80 x 86 computer equipped with the DOS operating system by simply typing CO2SYS at the prompt after loading the executable file CO2SYS.EXE.